Pallet storage next to a loading dock: condensation cycles

Fourteen degrees at six in the morning, twenty-six by two in the afternoon, and a roller door that opens sixty times a day. That is a warehouse breathing, and the bottom layer of every pallet within five metres of the door is breathing with it. In a polymer store, that cycle is the difference between a bag that flows in March and a bag that has to be broken up in July.

Condensation on stored goods is counter-intuitive. Water does not come through the roof; it comes out of the air when warm, humid air meets a cooler surface. The granules inside a bag are usually the coolest thing in the building after a cold night, so the moisture in the incoming afternoon air condenses on the bag, wicks through the paper or woven PP, and lands on the polymer.

The physics, in one paragraph

Air at 25 degrees and 70 percent RH holds about 16 g of water per cubic metre. Cool that air to 15 degrees and it can hold about 11 g, so 5 g per cubic metre has to go somewhere — usually onto the nearest cold surface. A dock door that opens onto a warm yard, with a store at 15 degrees inside, is a condensation machine running all summer. The effect is strongest at floor level, where the cold air pools and where the pallets sit.

You can watch it happen. On a humid morning, look at the bottom two layers of a pallet near the door. The stretch wrap will be fogged on the inside, the cardboard will be soft at the corners, and the pallet deck will be dark. That is the water that is heading for your granules.

Five metres of separation buys most of the fix

The cheapest intervention is layout. Keep polymer pallets at least five metres back from any door that opens to the outside, and never in the direct line of the door. In a store where that is impossible, a curtain strip or a fast-acting door pays for itself in one humid season, especially if the door is a dock leveller that stays open while a trailer is loaded.

Second is elevation. A plastic pallet or 100 mm of dunnage keeps the bag off a concrete slab that sits at ground temperature and wicks moisture upward. Third is orientation: stack with the bag seam facing up where possible, so any water that does land runs off rather than pooling in a fold.

Wrapping: more is not automatically better

Stretch wrap keeps rain off and keeps moisture in. A pallet wrapped while the granules are still warm from a delivery, then moved into a cool store, will condense on the inside of the film and hold that water against the bags for weeks. The same wrap on a pallet that has equilibrated overnight is harmless.

The practical rule is to let a delivery stand unwrapped for a shift before wrapping, and to use a breathable top sheet rather than a sealed cap if the store is humid. If the pallet is going into a container for export, that changes the calculation entirely and the wrap should be sealed with a desiccant inside — but that is a shipping decision, not a storage one.

Reading the damage before it becomes a complaint

A pallet that has taken on moisture announces itself in three ways. The bags at the bottom feel firmer and slightly heavier; a 25 kg bag that reads 25.4 kg has taken up 400 g of water, which is enough to crust the surface granules. The second is a faint stiffness when you squeeze the bag at the corner, where the granules have fused. The third is weight variation across the pallet, which is easy to spot if the store weighs incoming and outgoing bags as a matter of routine.

Once a bag has crusted, it is not necessarily spoilt. Break the crust by kneading the bag before use, or empty it into a mixer where the shear will do the job. But do not put a crusted bag into a volumetric feeder and expect an accurate dose, because the flow will be intermittent and the density reading will be wrong.

Five metres, one pallet of dunnage, and a weighbridge check once a month will handle most of this. If your store has a history of caking and you are trying to work out whether the material or the building is at fault, the storage section of the super absorbent powder guide sets out the humidity and temperature limits to test against.